Current aircraft exhibit three levels of piloting equipment:                a first level of equipment consisting of the flight controls acting directly on the control surfaces and engines,        a second level of equipment consisting of the automatic pilot and/or flight director acting on the flight controls, directly for the automatic pilot or by way of the pilot for the flight director, so as to slave the aircraft to a flight parameter such as for example, heading, roll, pitch, altitude, speed, etc, and        a third level of equipment consisting of the flight management computer known by the initials FMS derived from the expression “Flight Management System” capable of formulating a flight plan and of acting on the automatic pilot or the flight director so as to make the aircraft follow it.        
The FMS flight management computer, hereinafter called the FMS flight computer, has, among other main functions: the formulation and automatic tracking of a flight plan, a flight plan being composed of the lateral and vertical trajectories that must be followed by the aircraft to go from the position that it occupies to its destination, as well as travel speeds along these trajectories.
The formulation of a flight plan is done inter alia, on the basis of imposed waypoints optionally associated with time, altitude and speed constraints. These imposed waypoints and their associated constraints are introduced, into the FMS flight computer, by an operator of the aircraft, for example an aircraft crew member, by means of an item of equipment of the flight deck with keyboard and screen ensuring the man-machine interface such as that known by the abbreviation MCDU standing for: “Multipurpose Control and Display Unit”. The formulation of the flight plan proper consists in constructing the lateral and vertical trajectories of the flight plan on the basis of a chaining of “flight segments” LEGi (or “Legs”) each identified by a flight segment index i defining an order in the flight plan. The flight plan starts from a departure point, passes through imposed waypoints, and finishes at an arrival point, while complying with standardized construction rules and while taking account of time, altitude and speed constraints optionally associated with each imposed waypoint. A flight segment LEGi is delimited by a departure Waypoint WPTi−1 and an arrival Waypoint WPTi.
During an aircraft flight, the FMS plays a central role in the control of the trajectory. It is the nerve centre of this command. During navigation carried out under the command of the FMS, one speaks of navigation in “managed” mode: the FMS controls the automatic pilot and optionally the auto-throttle.
In “managed” mode, the leg LEGi which is followed by the aircraft is called the “active leg”: the aircraft then steers towards the arrival Waypoint WPTi, that is to say the end terminating the active leg.
According to a predefined criterion of proximity of the aircraft and of the arrival Waypoint of the active leg, a transition is carried out so as to follow the flight plan stage by stage. This transition consists in modifying the current active leg LEGi and in considering the leg which follows LEGi in the flight plan, that is to say LEGi+1, to be the new active leg.
Once the condition or conditions of the predefined criterion are fulfilled, the current active leg is modified: LEGi+1 then becomes the active leg, until the condition or conditions of the predefined criterion are again fulfilled with WPTi+1 and so on and so forth as the aircraft passes in proximity to the various Waypoints which make up the flight plan.
When an aircraft has interrupted a managed navigation mode, the flight plan is no longer necessarily followed by the aircraft. This is the case, for example, when piloting “with the stick” or when the aircraft is in “selected” mode: in this navigation mode, the aircraft pilot transmits lateral, and optionally vertical, displacement and speed setpoints directly to the automatic pilot and to the throttle. Thus, the aircraft may deviate considerably from the trajectory defined in the flight plan stored in the FMS.
When the pilot of the aircraft wishes to rejoin the trajectory of the flight plan, it is not always possible for him to rejoin the trajectory of the flight plan by passing back directly to managed mode. This difficulty may stem, for example, from a significant lateral gap between the aircraft and the current active leg. Specifically, in managed mode, the aircraft will have a tendency to rejoin the arrival Waypoint WPTi of the last active leg LEGi, which is not always possible.
Now, depending on the operating mode of the FMS, when a waypoint WPTi cannot be rejoined, neither can the subsequent waypoints of the flight plan and the active leg remains stuck on the leg terminating at WPTi.
The freezing of the active leg also has consequences in relation to air traffic control: specifically, an increasing number of surveillance systems currently call upon digital data links (Datalink). Aircraft regularly send the control centres the position of the aircraft as well as its two scheduled passing positions (ADS application, the initials standing for Automatic Dependent Surveillance) through digital data links. The freezing of the leg causes erroneous data to be transmitted to the air traffic control.
Additionally, an increasing number of aircraft are equipped with automatic report applications allowing the operational centres of airlines (AOC centres) to monitor, in real time, the utilization of aircraft. Thus, with each logging point sequencing, the aircraft sends an electronic message to its company. Here again, point non-sequencing makes it impossible for the company to follow in real time the route of the aircraft.
This difficulty can be solved through manual intervention by the pilot who directly modifies the flight plan by eliminating one by one the Waypoints situated upstream of the Waypoint WPTp chosen by the pilot to rejoin the flight plan.
This manual intervention by the pilot generates a considerable workload: in certain situations, the active leg is separated from the leg to be rejoined by a large number of legs in the flight plan, thereby consequently requiring a large number of manual eliminations of Waypoints.
Additionally, input errors by the pilot are always possible in particular in a case where the Waypoints have mutually close identifiers. This is the case for example for Waypoints during ocean crossings.
Finally, the manual interventions are currently performed “head-down” thereby requiring great vigilance on the part of the crew which is not always compatible with the piloting tasks.
The main defect therefore stems from the fact that there is no automatic mechanism allowing the FMS to return to navigation in managed mode, and therefore that the pilot is induced to carry out multiple operations with this aim.